1. Field of the Invention
This invention relates to a liquid crystal display device and, more particularly, a polysilicon thin film transistor type liquid crystal display device in which driver circuits may be integrated.
2. Related Art
There have been many types of proposed and/or commercialized liquid crystal display devices. Most of them have a liquid crystal layer, the material of which is typically twisted nematic liquid crystal. In the liquid crystal display device of this sort, a display can be performed in response to changes in twist of liquid crystal molecules, i.e., those in optical rotary power of the liquid crystal layer. In more detail, its operation principle is to control light passing through the liquid crystal display panel and reaching to an observing side by making use of optical properties of the liquid crystal layer, i.e., its rotary power or birefringence and linear polarization.
This liquid crystal display panel includes thin film transistors (called "TFTs") which switch to supply voltages to the liquid crystal layer of the pixels. The TFTs in commercial use or developed ones are made of amorphous silicon or polysilicon as a semiconductor material. Since the polysilicon has higher mobility than the amorphous silicon, the polysilicon TFTs have advantages resulting from it. First, it is permissible to provide more electric charge per unit time to the TFTs. As a result, the size of TFT is made smaller so that the aperture ratio at a pixel of the liquid crystal display device increases. Second, driver circuits for TFTs of the liquid crystal display device can be formed on the same substrate as the TFTs. Physically separated driver integrated circuits (called "ICs") and assembling the same with the panel are, thus, unnecessary so that the manufacturing cost for the liquid crystal display device becomes less expensive. The polysilicon driver ICs for the TFTs can easily comply with such a requirement for the ICs as formed on quite narrow frame edge portions of the liquid crystal display panel. The polysilicon TFTs attract a great deal of attention as an important technology because of the advantages as set forth above.
The color projector, above all, is generally a three-plate type with three color panels for three primary red, green and blue colors (called "R, G and B colors") to attain high brightness. The color video camera monitor is, however, a one-plate (panel) type with a color filter. Further, some projectors with the one panel diverted from the camera monitor use have been commercialized for low brightness applications.
The one-plate type color liquid crystal display device with the color filter requires pixels three times as many as the three-plate type device so that the aperture ratio of the former becomes smaller than the latter. In addition, optical loss takes place with the color filter so that it is quite difficult to put the one-plate type projector with high brightness into practical use. The mainstream color projector, therefore, has been the three-plate type. The projector of this type needs three liquid crystal display panels and optical separation and synthesizing system, and it is quite difficult to commercialize a less expensive three-plate color projector.
From a view point of less expensive commercialization, several new one-plate types of projectors are widely noticed. Some projectors of this type, which have been actively developed, include a liquid crystal display panel provided with dichroic mirrors and a micro-lens array for color separation and direction changes, or hologram optical elements (called "HOE") with both color separation and focusing functions. The latter includes a light source, optical systems to lead parallel rays to the panel, and a projection lens in addition to the HOE, as major elements, so that its optical system is greatly simplified and it is expected to be of less expensive cost. The HOE will be briefly described hereinafter but N. Ichikawa, "Holographic Optical Element for Liquid crystal Projector", the proceedings of the Asia Display Society, pages 727 through 729, 1995, for instance, is a reference article for that technology.
FIG. 17 is a perspective schematic view of a liquid crystal display device with the HOE to explain its operation principle. For the sake of convenience, pixels for only one set of R, G and B colors in the display device are illustrated in the drawing. As shown, the HOE 102 is disposed on the incident light side of the display device 104 which includes a TFT array substrate 105 and a counter substrate 106 provided opposite thereto. The HOE 102 is provided for each set of pixels corresponding to the R, G and B colors in the liquid crystal display panel. Parallel white light 103 from a light source is incident to each HOE 102 at the incident angle of about 40.degree.. The HOE 102 has diffraction and lens effects. Namely, the HOE 102 separates optical rays from the incident light, focalizes them on its focal plane, and forms a continuous spectrum distribution. Where the liquid crystal display panel 104 is properly disposed on the focal plane, color components are incident into the apertures 107, 108 and 109 of the pixels of R, G and B colors of the panel, respectively. In other words, the incident white light 103 on the HOE 102 is separated into the continuous spectrum but only components of the R, G and B colors 110, 111, and 112 pass through the apertures 107, 108, and 109 and become output light 115, 116, and 117. With this structure, the display device has advantages in that it performs a color display without any color filter and with no color filter loss and that its optical components are minimized in size and less expensive in cost.
A one-plate projector type liquid crystal display device using the HOE and the like needs pixels for R, G and B colors and the number thereof is three times that of pixels for a three-plate type one so that its definition must be much higher. In the case of the stripe-like color pixel arrangement shown in FIG. 17, the pixel length and breadth ratio is 3:1 and the lateral pitch is shorter. As a result, where its lay-out of a TFT and a storage capacitor in each pixel is the same as that of the conventional device with the ratio of 1:1, the TFT and the like remain disposed in an aperture of the pixel, and they are obstructions so that the aperture ratio decreases greatly. An explanation of this problem will be described in greater detail hereinbelow.
FIG. 18 is a plan view of a TFT array substrate used in a conventional liquid crystal display device with the pixel length and breadth ration is approximately 1:1. In the display device, a video signal provided to a signal line 134 is supplied to pixel electrodes 137A and 137B, respectively, through a source contact 133 of a TFT, gate portions 130A and 130B and pixel electrode contacts 140A and 140B. Selection of the gate portions 130A and 130B is controlled by scanning pulses supplied to scanning lines 139A and 139B. A storage capacitor 132 is defined between a storage capacitor line 135 and a polysilicon layer 131 to hold the video signal supplied to each pixel electrode.
The gate portion 130A and a part of the storage capacitor portion 132 are disposed under the signal line 134 and the storage capacitor line 135 is commonly provided between neighboring upper and lower pixels in the display device as shown in FIG. 16. 3 .mu.m wiring rule applied to this layout achieves 40 .mu.m pixel length with the aperture ratio of 36%. In this structure, however, drain contacts of the TFTs, i.e., pixel electrode contacts 140A and 140B are formed in the vicinity of the centers in the approximately rectangular apertures, respectively. If this structure is used for the rectangular, shorter width pitch pixels, the pixel electrodes protrude to the center portions of narrow apertures. The strongest intensity one of light components passing through each pixel is blocked if the display device of that structure is used in combination with the HOE and the micro-lens array.
As explained above, the incident white light on the HOE is separated, focalized, and, eventually, formed a continuous spectrum distribution on its focal plane. Each pixel for R, G or B color is desirably provided with a uniformly shaped aperture at a place corresponding to the R, G or B color spectrum distribution. In the case, however, that light blocking plates of the electrode contacts protrude to the pixel aperture as shown in FIG. 16 and set forth above, and a separation among pixels is insufficient, the pixel is difficult to receive effectively the pure R, G or B color light only and its color purity decreases. It is quite difficult to provide a high performance display panel by means of a conventional HOE one-plate type liquid crystal display device with the structure mentioned above.
Further, in the cases that a pixel is narrow and its width in the direction of a scanning line and a storage capacitor line is shorter than its length in the direction of a scanning line, and that it is necessary to make contact holes in source and drain portions of a device like a polysilicon TFT, it is more difficult to dispose TFTs within the pixel pitch in the lateral direction as the pixel definition becomes finer. It is also difficult to increase the capacitance value of the storage capacitor because the pixel pitch is small.
In addition, with the conventional structure shown in FIG. 18, since two neighboring scanning lines are disposed between the narrow pixels, short-circuit troubles take place easily. If the common storage capacitor line 135 as shown is not used, the storage capacitor line and the scanning line are provided in parallel with each other in a narrow space between the pixels and it also brings about short-circuit troubles between the. If the space is made large enough to avoid such problems, the aperture ratio becomes much poor.